Dictionary comprehension with condition in Python - Mini Project: Build & Apply

Dictionary Comprehension with Condition

📖 Scenario: You are managing a small bookstore's inventory. You have a list of books with their prices. You want to create a new dictionary that only includes books priced below a certain amount to plan a discount sale.

🎯 Goal: Build a dictionary comprehension with a condition to filter books priced below a set threshold.

📋 What You'll Learn

Create a dictionary called books with given book titles and prices.

Create a variable called max_price to set the price limit.

Use a dictionary comprehension with a condition to create a new dictionary called discount_books with books priced less than max_price.

Print the discount_books dictionary.

💡 Why This Matters

🌍 Real World

Filtering products or items based on price or other conditions is common in online stores and inventory management.

💼 Career

Knowing how to filter dictionaries efficiently helps in data processing, reporting, and building features like price filters in apps.

1

Create the books dictionary

Create a dictionary called books with these exact entries: 'Python Basics': 45, 'Data Science 101': 60, 'Machine Learning': 55, 'Deep Learning': 70, 'AI for Beginners': 40.

Python

# Create the books dictionary with given titles and prices
# Your code here

Hint

Use curly braces {} to create a dictionary with keys as book titles and values as prices.

2

Set the maximum price

Create a variable called max_price and set it to 50.

Python

books = {'Python Basics': 45, 'Data Science 101': 60, 'Machine Learning': 55, 'Deep Learning': 70, 'AI for Beginners': 40}
# Set max_price to 50
# Your code here

Hint

Just assign the number 50 to the variable max_price.

3

Create filtered dictionary with comprehension

Use a dictionary comprehension to create a new dictionary called discount_books that includes only the books from books with prices less than max_price. Use for title, price in books.items() in your comprehension.

Python

books = {'Python Basics': 45, 'Data Science 101': 60, 'Machine Learning': 55, 'Deep Learning': 70, 'AI for Beginners': 40}
max_price = 50
# Create discount_books dictionary with comprehension and condition
# Your code here

Hint

Use the format {key: value for key, value in dictionary.items() if condition} to filter.

4

Print the filtered dictionary

Print the discount_books dictionary using print(discount_books).

Python

books = {'Python Basics': 45, 'Data Science 101': 60, 'Machine Learning': 55, 'Deep Learning': 70, 'AI for Beginners': 40}
max_price = 50
discount_books = {title: price for title, price in books.items() if price < max_price}
# Print the discount_books dictionary
# Your code here

Hint

Use the print function to show the dictionary on the screen.

Practice

1.

What does the following dictionary comprehension do?
{k: v for k, v in {'a': 1, 'b': 2, 'c': 3}.items() if v > 1}

easy

A. {'a': 1, 'b': 2, 'c': 3} - keeps all items

B. {'a': 1} - keeps items with values equal to 1

C. {'b': 2, 'c': 3} - keeps items with values greater than 1

D. {} - removes all items

Solution

1. Step 1: Understand the dictionary comprehension structure

   The comprehension loops over each key-value pair in the original dictionary.

2. Step 2: Apply the condition if v > 1

   Only pairs where the value is greater than 1 are included in the new dictionary.

3. Final Answer:

   {'b': 2, 'c': 3} - keeps items with values greater than 1 -> Option C

4. Quick Check:

   Filter values > 1 = {'b': 2, 'c': 3} [OK]

Hint: Look for the condition after the for loop to filter items [OK]

Common Mistakes:

• Ignoring the condition and including all items
• Confusing keys and values in the condition
• Using wrong comparison operator

2.

Which of the following is the correct syntax for a dictionary comprehension with a condition?

?

easy

A. {k: v for k, v in d.items() if v % 2 == 0}

B. {k: v if v % 2 == 0 for k, v in d.items()}

C. {k: v for k, v in d.items() where v % 2 == 0}

D. {k: v for k, v in d.items() when v % 2 == 0}

Solution

1. Step 1: Recall dictionary comprehension syntax

   The correct syntax is {key: value for key, value in iterable if condition}.

2. Step 2: Identify the correct option

   {k: v for k, v in d.items() if v % 2 == 0} matches the correct syntax with if after the loop. Others use invalid keywords or wrong order.

3. Final Answer:

   {k: v for k, v in d.items() if v % 2 == 0} -> Option A

4. Quick Check:

   Correct syntax uses 'if' after for loop [OK]

Hint: Remember: condition goes after the for clause in comprehension [OK]

Common Mistakes:

• Placing 'if' before the for loop
• Using 'where' or 'when' instead of 'if'
• Incorrect order of clauses

3.

What is the output of this code?

nums = {'x': 10, 'y': 5, 'z': 0}
filtered = {k: v for k, v in nums.items() if v}
print(filtered)

medium

A. {'x': 10, 'y': 5}

B. {}

C. {'z': 0}

D. {'x': 10, 'y': 5, 'z': 0}

Solution

1. Step 1: Understand the condition if v

   This condition filters out values that are 'falsy' in Python, like 0, None, or empty.

2. Step 2: Apply the condition to each item

   Values 10 and 5 are truthy, so included; 0 is falsy, so excluded.

3. Final Answer:

   {'x': 10, 'y': 5} -> Option A

4. Quick Check:

   Falsy values excluded = {'x': 10, 'y': 5} [OK]

Hint: Falsy values like 0 are skipped when condition is just 'if v' [OK]

Common Mistakes:

• Including zero as truthy
• Confusing keys and values
• Expecting all items to be included

4.

Find the error in this dictionary comprehension:

data = {'a': 1, 'b': 2, 'c': 3}
result = {k: v for k, v in data.items() if v > 1 else 0}
print(result)

medium

A. No error, outputs {'b': 2, 'c': 3}

B. SyntaxError due to incorrect use of else in comprehension

C. Outputs {'a': 0, 'b': 2, 'c': 3}

D. KeyError because of else clause

Solution

1. Step 1: Check the use of else in comprehension condition

   Dictionary comprehensions cannot have an else clause directly after the if condition.

2. Step 2: Identify correct syntax

   To use else, it must be inside a value expression with a ternary operator, not after if.

3. Final Answer:

   SyntaxError due to incorrect use of else in comprehension -> Option B

4. Quick Check:

   else must be inside value expression, not after if [OK]

Hint: Use ternary inside value, not else after if in comprehension [OK]

Common Mistakes:

• Placing else after if in comprehension
• Confusing ternary operator syntax
• Expecting else to filter keys

5.

You have a dictionary of student scores:
scores = {'Alice': 85, 'Bob': 42, 'Charlie': 73, 'David': 58}
Use dictionary comprehension with a condition to create a new dictionary passed containing only students who scored 60 or more, but store their scores as 'Pass' instead of the number.
Which code correctly does this?

hard

A. {k: 'Pass' for k, v in scores.items() if v > 60}

B. {k: 'Pass' if v >= 60 else 'Fail' for k, v in scores.items()}

C. {k: v for k, v in scores.items() if v >= 60}

D. {k: 'Pass' for k, v in scores.items() if v >= 60}

Solution

1. Step 1: Understand the requirement

   We want only students with scores 60 or more, and their values replaced by 'Pass'.

2. Step 2: Check each option

   {k: 'Pass' for k, v in scores.items() if v >= 60} filters with if v >= 60 and sets value to 'Pass'. {k: 'Pass' if v >= 60 else 'Fail' for k, v in scores.items()} includes all students with ternary but no filtering. {k: v for k, v in scores.items() if v >= 60} keeps original scores. {k: 'Pass' for k, v in scores.items() if v > 60} excludes 60 exactly.

3. Final Answer:

   {k: 'Pass' for k, v in scores.items() if v >= 60} -> Option D

4. Quick Check:

   Filter and replace value with 'Pass' = {k: 'Pass' for k, v in scores.items() if v >= 60} [OK]

Hint: Filter with if, set value directly to 'Pass' in comprehension [OK]

Common Mistakes:

• Using else without filtering
• Not including scores equal to 60
• Keeping original scores instead of 'Pass'